High efficiency permanent air filter and method of manufacture

ABSTRACT

PCT No. PCT/US97/18482 Sec. 371 Date Sep. 18, 1998 Sec. 102(e) Date Sep. 18, 1998 PCT Filed Oct. 15, 1997 PCT Pub. No. WO98/17368 PCT Pub. Date Apr. 30, 1998An air filter for use in residential and commercial heating and air conditioning systems including a laminated unit (100) having a filter media (102) where either side of the filter media (102) has a porous layer (104, 106) of passive electrostatic netting. The air filter has a corrugation configuration (142, 144) to increase the available surface area when compared to conventional flat filters.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national stage application of InternationalApplication No. PCT/US97/18482, filed on Oct. 15, 1997, which claims thebenefit of U.S. Provisional Application Ser. No. 60/028,786, filed onOct. 18, 1996.

TECHNICAL BACKGROUND

The present invention relates to filters cleanable by washing orvacuuming or the like for inlet air heating and air conditioning systemsused in residential or commercial buildings and, more particularly, thisinvention relates to an electrostatic filter for such systems having anextended service life.

BACKGROUND ART

Inlet air filters for heating and air conditioning systems have been inuse for decades. The inlet air or primary filter was originally intendedto protect the heating coils and mechanical devices such as fans fromdamage by airborne particles. As the harmful effect of inhaled particleson human health became known, inlet air filters were designed removethis particulate matter from the air.

The most popular prior art configuration is a thin, rectangular,disposable filter. The filter contains fiberglass, animal hair, fibrousfoam or polymeric media or aluminum mesh encased in a cardboard orplastic frame. Prior art filters can comprised polyester panels,urethane foam or latex coated animal hair. Recently, prior art filterscontaining static or passive electrostatic media have become available.

Most residential resistance specifications require air filters to havean initial pressure drop of no more than 0.22 inches of water for an airflow rate of 300 feet per minute. The dust spot efficiency for typicalprior art air filters tested using ASHRAE 52.1-1992 is 20% or less. TheASHRAE efficiencies of four types of commercially available filtersfollows:

                  TABLE 1                                                         ______________________________________                                                      Area of Media                                                   Filter Type   Square Feet                                                                              ASHRAE EFFICIENCY                                    ______________________________________                                        Fiberglass Throwaway                                                                        4.0        <20%                                                 Electrostatic (Passive)                                                                     4.0        20%                                                  Electrostatic (Fibers)                                                                      Up to 8.9  30%                                                  Pleated Polyester Blend                                                                     Up to 6.9  25%                                                  ______________________________________                                    

When these filters are first placed in use across an air stream, theyhave a very low filtering efficiency. Typically, the exhaust or dustspot efficiency is about 8%. However, as dust particles are collected,the dust collection efficiency can increase to approximately 20%. Atthis point, the filter is ready to be replaced. Prior art filters areusually difficult to clean since the dust particles become embedded inthe media.

Another current concern is the recognition that particles below 10microns are not filtered by the cilia hairs in the nose and aretherefore inhaled into a human lung. Fibrous particles such as asbestosand fiberglass are known to cause respiratory diseases. Most currentinlet air filters for heating and air conditioning systems are not veryefficient in capturing these small particles. Filters containing layersof electrostatic media perform better within this range of particles butthese filters also become clogged. Since they cannot be efficientlycleaned they must be replaced.

DISCLOSURE OF THE INVENTION

It has been discovered in accordance with the invention that the angleof approach of the air stream effects the particle capture efficiency,the life of the filter between cleaning and the difficulty of cleaningthe filter. The filter of the invention causes a non-perpendicular pathof air flow resulting from the corrugated or pleated configuration ofthe novel air filter. Because of the corrugated configuration, moresurface area exists than with a standard, flat, prior art air filter.The increased surface area of the novel filter also results in a lowerpressure drop than experienced with prior art electrostatic air filters.The filter has significantly longer life between cleanings and it isvery easy to clean. The novel air filter's efficiency for removingparticulate matter is also an improvement over the prior art.

The purpose of the invention is to provide an upstream passiveelectrostatic layer disposed upon a filter media layer wherein bothlayers are in a corrugated or pleated configuration to increase filtersurface area when compared to a flat surfaced air filter.

A number of embodiment configurations can be utilized which incorporatemy novel concept. The following are examples.

Two Layer Embodiment. A filter media will be comprised of fibers of aplastic material such as polyester or nylon held together with binder,preferably an acrylic thermo setting binder. The filter media isselected from material which can capture in excess of 80% of particulatematter of 10 micron or less in size. The filter media layer ispreferably made from polyester having 6-15 Denier fibers, an acrylicresin binder preferably containing an antimicrobial agent. The media maycontain a curable resin such as a thermosetting, light cured or wateractivated resin. The resin may be dispersed throughout the media asapplied as stripes to the media at locations corresponding to the topsof the pleats, before or after pleating, preferably after pleating. Theresin is then cured to a rigid state to hold the pleats in shape. Such athermal-set filter media, in combination with a passive electrostaticlayer can be corrugated without the need of metallic grills to maintainthe corrugated shape. The desired passive electrostatic layer can bedescribed as a netting material. The electrostatic layer may be madefrom media such as polypropylene, polyester, nylon or polycarbonate.More preferably, the electrostatic layer is made from DELNET RB0404-12P,a product of Applied Extrusion Technology, Middletown, Del. orEquiliuent. In use, the air filter would be installed with theelectrostatic layer facing upstream. In this embodiment, stitching,stapling, thermal welding, adhesives or other means could be utilized toattach the filter media and electrostatic layer to one another.Additionally, a second electrostatic layer may be disposed on thedownstream side of the filter media to increase the filter's efficiency.

Five Layer Embodiment. In this configuration, either side of the filtercan be placed upstream.

This embodiment has a central filter media layer from the same materialas described above although it is not required to have a thermal-settingproperty.

Disposed on either side of the central layer is a passive electrostaticlayer of the same material described earlier. Disposed on the outwardfacing surface of each electrostatic layer is a grill, suitably formedof expanded metal. The grill is preferably made from metal. Mostpreferable, the grill is made from a non-corrosive material such asaluminum. A non-metallic material may also be utilized as a grill. Thepurpose of the grill is to provide a deformable material which willcause the filter, once corrugated, to maintain its corrugated shape. Thegrill layers is required when a resilient filter media is used whichwill tend to return to a flat shape.

When it was attempted to form the electrostatic layers and central layerinto a corrugated configuration having angled surfaces, the materialcomprising these layers have a resilient property which would attemptreturn to a flat configuration as soon as the forming tool was releasedfrom the surface. A further aspect of this invention is a method offorming flat media into an angled surface in a manner to prevent it fromreturning to its original configuration.

In one alternative of the five layer embodiment, the electrostatic mediais placed between sheets of expanded metal screen or grill. The edge ofat least one screen is wrapped over the opposing edge to lock theassembly together and restrain the media from expanding and returning toits original shape.

In a second alternative of the five layer embodiment, the electrostaticmedia layers, the central layer and the outside grills are stitched toeach other.

In a third alternative of the five layer embodiment, a respective grilland electrostatic layer are adhesively bonded to one another. During theassembly process, a central layer is disposed between two layers oflaminated grill and electrostatic media.

The grill may be easily washed. Either side of the grill may facedownstream provided the grill layer is the outside layer. It has beendiscovered that if the grill is positioned between the electrostatic andcentral layers, adhesive may be used to bond the grill to both layers.This has the advantage of overcoming the resilient tendency of thecentral layer to become flat. The grill however, may only be disposedbetween both layers on the upstream side. Testing has shown that whilethe air flow tends to force the electrostatic layer into the grill andcentral layer on the upstream side, on the downstream side air flowtends to displace the electrostatic layer away from the central layer.Therefore, it is preferred to position the grill layer as the outsidelayer on the downstream side.

My novel air filter may be made into any size. However, as the size ofthe filter increases, it is preferred that the gauge of the grill layersincrease. Because of the resilient tendency of the central layermaterial as described earlier, the larger the air filter, heavier gaugematerial is required to maintain the filter's corrugated shapeespecially in the central surface area of the filter.

Another feature of the invention is the use of two sheets of expandedmetal. The prior art used expanded metal grills only for cleanable,washable metal filters. In the invention the expanded metal layers arebent into an angled form at the same time as the electrostatic media andcentral layer. The metal layers prevent the angled electrostatic mediaand central layer from returning to a flat form. The metal layers havevery wide openings and thin strands which do not contribute to capturingparticles but also do not contribute to pressure drop of the compositemedia.

The filter of the invention is optimally angled to capture particles. Itcan be cleaned by shaking and/or by washing. The media has low pressuredrop and high efficiency for the range of particles experienced with theinlet air. Because it can be cleaned and reused, my invention can beconsidered a permanent filter.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is cross sectional view of a multilayer filter media folded orpleated to increase surface area to 160% of the flat media;

FIG. 2 is another cross section of the filter of FIG. 1, shown inservice;

FIG. 3 is a schematic graph showing filter efficiency;

FIG. 4 is a graph showing the efficiency of folded electrostatic filtersof the invention in the 0.3 to 10 micron range;

FIG. 5 is a graph showing the efficiency of flat commercialelectrostatic filters in the same range tested under the sameconditions;

FIG. 6 is a schematic view of one embodiment of the novel 5 layer mediaair filter;

FIG. 7 is a schematic view of one embodiment of the novel 5 layer mediaair filter after securing the ends;

FIG. 8 is an end view in elevation of the roller for corrugating themedia in one embodiment of the invention;

FIG. 9 is a schematic view of an embodiment of a continuous system forforming the filter media of the invention; and

FIG. 10 is a schematic view of another embodiment of a continuous systemfor forming filter media of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The passive electrostatic filter contains a core layer of a bendablematerial, from 0.05 to 0.45 inches in thickness selected from polymericfoam or a high loft fibrous polymeric material such as air laidpolyester fibers having a density of from 0.5 to 5 ounces per squareyard (OPSY). The fibers are resin bonded by a resin such as an acrylicresin or point bonded or needle bonded. In a preferred embodiment thecentral or core layer is comprised of a thermal set material. Thismaterial removes most of the particles from the air stream, typically atleast 80% of the incoming particles having sizes up to 10 microns.

The particle capture efficiency of the core material is enhanced byplacing a layer of electrostatic polymer material on each side of thecore material. The material can be woven or non woven. More preferably,passive electrostatic materials are used which become charged as airflows past the polymers that tend to have natural static charges. Theseare preferred since the available active electrostatic fibrous materialsproduce an undesirably high pressure drop as a filtration media and arenot cleanable. The passive material is usually formed by casting,extrusion or weaving from a polymer of a nonsaturated alkene monomerhaving 2-8 carbon atoms. One type of material is woven polypropylenenetting having a thickness from 0.01 to 0.10 inches, usually from 0.02to 0.06 inches. The yarn diameter can be from 1-35 denier, preferablyfrom 5-20 denier. The ratio of warp to fill yarn is from 1.2/1 to 3/1and the netting is woven in a manner to resemble a honeycomb structure.A commercially available material is a honeycomb weave netting of 8 milthick polypropylene fibers available in thicknesses of 0.03 and 0.05inches. The yarn count is 51 warp and 32 fill.

The outside grill layer does not contribute to filtering but is presentto hold the angled media after it has been formed. Again, the layer mustbe capable of being easily bent or deformed during the assembly process.If the sheet of material is too thick it is difficult to bend and if itis too thin, it is not capable of holding the 3 layer media in its bentshape. Expanded metal such as steel or aluminum having a thickness from0.010 to 0.25 inches, performs satisfactorily in the filter of theinvention. The most preferable gage thickness is dependent upon the sizeof the air filter. Larger air filters will require a more sturdy orheavier gage grill. The expanded metal grill should have open area of atleast 70%. Open area can be provided by thin strands from 0.01 to 0.18in thickness expanded to rectangular or parallelogram openings having anarea of at least 0.5 square inches.

In the design of the filter of the invention, the following five methodsof capturing particles were considered in providing a filter having theoptimum combination of high efficiency with the lowest possibleresistance:

1. Straining or screening

2. Interception

3. Impaction or impingement

4. Diffusion effects

5. Electrostatic attraction

EXAMPLE 1

The exterior layers on either side of the air filter media are comprisedof expanded aluminum having a thickness of 0.040 inches and strandthickness of about 0.04 inches. The rectangular openings were 1.5×0.75inches.

The passive electrostatic layers adjacent each exterior layer iscomprised of polypropylene netting having a thickness of 0.03 inches,warp and fill yarn diameters of 8 mil, a weight of 3 oz/yard and a yarncount of 51 warp yarns per inch and 32 fill yarns per inch.

The center layer disposed between the electrostatic layers is a stiff,high loft polyester having the following properties:

    ______________________________________                                        Weight (ounces per square yard):                                                                    4.5 +/- 10%                                             Gauge (inches):       0.25 +/- 5%                                             Fiber Content:        Polyester                                                                     (6 & 15 Denier)                                         Binder Type:          Acrylic Latex                                                                 (solids 38%)                                            Porosity (cfm/Ftsq.@0.5WG):                                                                         740 cfm                                                 Color:                White                                                   Texture:              Stiff/Lofty                                             Antimicrobial:        Aegis                                                                         or Equiliuent                                           ______________________________________                                    

The effect of the angle of the media to particle capture efficiency wastested by placing a flat panel of the 5-layer filter media of Example 1across an air steam flowing at 300 feet per minute (FPM). Particles atdifferent sizes were counted with a Hiac/Rayco laser particle counterboth before and after the filter to establish efficiencies at differentmicron sizes. The angle of the filter corrugation was changed in 10degree increments from 0 degrees (flat) to 90 degrees.

                  TABLE 2                                                         ______________________________________                                        AVERAGE EFFICIENCY                                                                  0.3      0.5     1.0    3.0   5.0    10.0                               ANGLE Micron   Micron  Micron Micron                                                                              Micron Micron                             ______________________________________                                        FLAT  1.025    4.025   22.0   48.0  62.45  67.72                              10°                                                                          1.4      5.6     34.0   53.0  65.7   69.2                               20°                                                                          1.6      5.5     34.0   66.0  80.2   83.0                               30°                                                                          1.6      5.8     27.0   64.0  74.8   77.6                               40°                                                                          1.7      6.7     27.0   65.0  73.8   80.9                               50°                                                                          1.6      8.6     28.0   64.0  74.3   81.7                               60°                                                                          1.775    6.15    29.25  68.75 76.48  84.58                              70°                                                                          1.875    5.55    28.5   66.0  75.15  85.85                              80°                                                                          2.9      5.9     22.0   51.0  64.6   80.9                               90°                                                                          1.0      5.25    21.0   49.0  60.9   71.8                               ______________________________________                                    

The experiment indicates that the optimum efficiencies are with thefilter at an angle to the incoming air from 20-75 degrees, preferablyfrom 40 to 60 degrees.

The angled surface supplies an increase in efficiency. The resistance toair flow can be lowered by increasing the surface area of the filtersuitably by alternate bending of the panel to form opposed angledsurfaces. Most filters operate at low efficiency by straining aloneuntil enough dust particles are captured. As the filter fills withlarger particles, its efficiency in capturing small particles that candeposit in the respiratory system increases. The ability of a filter toinitially capture small particles is improved by adding electrostaticmedia to the filter, especially when the filter is positioned at anangle greater than 10 degrees or less than 80 degrees to the incomingair.

Referring now to FIG. 1, increased area and angled surface are bothprovided by corrugating the media 10 to form folds 12 having alternatingupper and lower outer curved ends 14, 16 and opposed angled walls 18,20. The ratio of the length of the sum of the two angled surfaces is atleast 1.2 to 2.5 times the length of the distance between adjacentcurved ends and preferably from 1.4 to 2.0. The joinder 24 of the innerwalls can form a radius.

Referring now to FIG. 2, as the incoming air 11 contacts the foldedmedia 10, the air will diffuse across the slanted wall surfaces 18, 20.The air will search for the path of least resistance to pass through thefilter 10. In the beginning, the path of least resistance will be theslanted wall surfaces 18,20 rather than at the inner ends 26. As shownin FIG. 2, the particles 30, especially those having larger diameters,will begin to collect at the bottom of the fold.

The air flowing to the bottom of the fold will be deflected changing itsdirection. The air, carrying smaller particulate matter gradually flowsup the angled walls 18, 20 of the fold. The walls will become loadedwith small dust and sub micron sized particles. The larger particleswhich travel at a higher velocity and inertia will collect at the bottomincreasing the efficiency for collecting sub micron sized particles. Thesame mechanism will occur on the side walls as the bottoms of the foldsload with dust.

All of the mechanisms of filtration are enhanced. Straining andelectrostatic deposition become more effective due to increased dwelltime. The deflection of the air increases the turbulence of the air flowenhancing impaction and interception of both large and small particles.Diffusional mechanisms are always in effect capturing sub-micronparticles.

The distance between adjacent outer ends 14 and 16 controls thethickness of the filter. The final filter assembly including the outsideborder frame has a thickness from 0.5 to 4 inches, usually about 0.75inch thick.

Experiments

Five filters about 2 feet wide and one inch thick prepared in accordancewith Example 1. Each were tested for dust collecting efficiency in the0.3 micron to 10.0 micron range from an air steam flowing at 300 feetper minute. The air temperature was 70.2° F. and the relative humiditywas 55%. Testing was by the laser counting system previously described.

The test results are as follows:

    ______________________________________                                        1.         Complete filter number 1                                                      0.3 micron     6.8%                                                           0.5            12.3%                                                          5.0            86.4%                                                          10.0           91.63%                                              2.         Complete filter number 2                                                      0.3 micron     7.2%                                                           0.5            14.3%                                                          5.0            83.92%                                                         10.0           94.28%                                              3.         Complete filter number 3                                                      0.3 micron     6.7%                                                           0.5            15.7%                                                          5.0            85.6%                                                          10.0           90.89%                                              4.         Complete filter number 4                                                      0.3 micron     6.8%                                                           0.5            14.6%                                                          5.0            88.6%                                                          10.0           93.51%                                              5.         Complete filter number 5                                                      0.3 micron     7.3%                                                           0.5            13.4%                                                          5.0            84.9%                                                          10.0           92.66%                                              ______________________________________                                    

The resistance was 0.15 W.G. and the average of the 5 tests are tabledbelow as well as the average efficiency of a typical flat commercialelectrostatic filter wherein the resistance was 0.20 W.G. or 33%greater.

    ______________________________________                                                                    PRIOR ART AIR                                     PARTICLE SIZE                                                                             NOVEL AIR FILTER                                                                              FILTER                                            (Micron)    EFFICIENCY (%)  EFFICIENCY (%)                                    ______________________________________                                        0.3         6.8             2.8                                               0.5         14.1            4.4                                               5.0         85.9            83.7                                              10.0        93.6            91.3                                              ______________________________________                                    

FIG. 4 is a graphical representation of the above results for my novelair filter design. FIG. 5 depicts the results for a flat prior art airfilter.

In comparing my novel air filter design to the prior art, at the 0.30micron range the average amount of particles observed by the laserupstream of the filter is 750,000 particles per minute. A prior artfilter efficiency of 2.8% would permit more than 729,000 particles perminute to pass through. The radial pleat having an efficiency of 6.80%would permit 30,000 fewer particles per minute.

The particle size that has the largest filtration efficiency differenceis at one micron. For this size, an upstream laser observedapproximately 12,000 particles per minute. With the prior art filterhaving a 22% efficiency, more than 9,360 particles per minute would passthrough the filter. However, my novel filter design has an approximateefficiency of 37% for filtering one micron sized particles and wouldpermit less than 7,560 particles per minute to pass through my filterdesign.

The radial pleat or corrugated air filter has an increased surface areaover the prior air flat filter. Its ease of cleaning can be attributedto the filters ability to let the dust collect between the folds and notenter the filter media itself. The filter will have a larger dustholding capacity and will last longer between cleaning. As the filtercollects dust, the filter efficiency will increase.

Another aspect of the invention is in the fabrication of the filters. Aspreviously discussed, one embodiment is a five layer assembly 100 asshown in FIG. 6. The central layer 102 is a thin, bendable foam or highloft polyester layer. The central layer 102 is covered with layers 104,106 of the passive electrostatic netting. The outer open mesh expandedmetal layer grills 108, and 110 complete the stack. One grill layer,either 108 or 110 is slightly longer at 114 and 116 by 1/8 to 1/2 inchon each side.

The longer length of one grill layer folds over the ends of thecorrugations and extends over and forms a mechanical bond 130 with theupper expanded metal layer 108. The locked flat assembly 132 preventsthe folds from returning to flat condition. After corrugation, thefilter is completed by securing a U-shaped metal channel, not shown,such as aluminum, to the four edges of the folded assembly. An adhesive,preferably water-based, can be applied to the U-shaped channel tofurther secure the filter in place thereby preventing the corrugatedfolds from returning to a flat condition and dislodging from the frame.

Referring again to FIG. 8, the rollers 120 contains a plurality ofoffset V-shaped forming members 122 evenly spaced on the surface 124 ofthe roller 120 a distance apart sufficient to form a curved radius inthe media at the bottom of a fold. The sharp, pointed tops 126 of theforming members 122 form the inner pointed ends of the 5-layer foldedassembly.

The filter assembly of the invention is suitable for replacement offilters used for filtering incoming air supplied to residential heatingand air conditioning systems. The filters usually have a thickness from0.5 inch to 3 inches, typically 1 to 2 inches and are rectangularusually having a width of 1 to 3 feet and a length of 1 to 3 feet.

A 5-layer filter assembly according to FIG. 9 and Example 1 and a flatconstruction competitor's filter were tested by an independent testlaboratory at a test velocity of 300 feet per minute. Results follow:

                  TABLE                                                           ______________________________________                                        Particle Size                                                                              EFFICIENCY                                                       (MICRON)     COMPETITION INVENTION                                            ______________________________________                                        0.3/0.5       0.8%        5.6%                                                0.7/1.0       0.4%       21.2%                                                1.0/2.0      12.7%       42.4%                                                2.0/3.0      40.7%       73.1%                                                3.0/5.0      55.7%       83.9%                                                5.0/1.0      56.2%       88.1%                                                ______________________________________                                    

The filter of the invention shows substantially better captureefficiency at all particle sizes. The filter of the invention willcapture substantially all particles that affect respiratory or allergicreactions.

It is to be realized that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

What is claimed is:
 1. A washable air filter for filtering inlet air toa heating and/or air conditioning system comprising an assembly formedof:a deformable, non-electrostatic pad of a high-loft, air laid, resinbonded polymeric fibers having a high filtering efficiency for 10 micronand less particles and having a thickness from 0.05 to 0.45 inches and adensity from 0.5 to 5 ounces per yard, said pad having a front surfaceand a rear surface; a layer of woven, porous, passive electrostaticfabric disposed on either side of the pad having an inside surface incontact with said surfaces, and having an outside surface; a deformablemetal grill disposed in contact with the outside surfaces of the fabric;and said assembly having a pleated configuration in which the anglebetween adjacent wall surfaces of the pleats is from 20° to 75°.
 2. Anair filter according to claim 1 in which the passive electrostaticfabric is a woven fabric selected from the group consisting of apolyalkylene of 1-8 carbon atoms.
 3. An air filter according to claim 2in which the passive electrostatic fabric is a woven net.
 4. An airfilter according to claim 2 in which the passive electrostatic wovenfabric comprises polypropylene.
 5. An air filter according to claim 4 inwhich the woven fabric has a thickness of from 0.01 inch to 0.1 inch. 6.An air filter according to claim 2 in which the polymeric fibers arepolyester.
 7. An air filter according to claim 6 in which the bondingresin is an acrylic resin.
 8. An air filter according to claim 7 inwhich the filter has a periphery and further including a stiff frameengaging said periphery.
 9. An air filter according to claim 8 in whichsaid filter is stitched about said periphery.
 10. A filter according toclaim 3 in which the thickness of the net material is from 0.02 to 0.06inches.
 11. A filter according to claim 10 in which the woven net has aweave which appears to have a honeycomb structure.
 12. A filteraccording to claim 1 in which the filter has an efficiency for 0.3micron particles of at least 6.8%.
 13. A method of filtering inlet airto an air conditioning and heating system with a permanent filtercomprising the steps of:placing a washable filter as defined in claim 1in the path of the inlet air; capturing particles from the inlet aironto the filter pad; removing the filter from the path of the inlet air;washing the filter pad to remove the particles; and repositioning thefilter in the path of the inlet air stream.
 14. A method ofmanufacturing a washable filter for filtering inlet air flow to aheating and/or air conditioning system, comprising the steps of:placinga sheet of passive electrostatic netting woven from polyalkylene fibersin contact with the surfaces of a non-electrostatic pad of a high-loft,air laid, resin bonded, polymeric fibers having a thickness from 0.05 to0.45 inch and a density from 0.5 to 5 ounces per yard and a highefficiency for 10 micron or less particles; placing a layer of porousmetal in contact with the outer surfaces of the netting to form anassembly; and pleating said assembly to form pleats in which the anglebetween wall surfaces is from 20° to 75°.
 15. A method according toclaim 14 in which the passive electrostatic netting is a woven fabricselected from the group consisting of a polyalkylene of 1-8 carbonatoms.
 16. A method according to claim 15 in which the passiveelectrostatic woven fabric consists of polypropylene.
 17. A methodaccording to claim 16 in which the woven fabric has a thickness of from0.01 inch to 0.1 inch.
 18. A method according to claim 17 in which thepolymeric fibers are polyester.
 19. A method according to claim 18 inwhich the bonding resin is an acrylic resin.
 20. A method according toclaim 14 in which said filter has a periphery and further including thestep of encasing said periphery in a metal frame.
 21. A method accordingto claim 20 further including the step of stitching said filter aboutsaid periphery.